# Why is the CO2 level lower in my house than outside?

Summary:
I purchased a high accuracy IR co2 sensor. Outside it reads ~400ppm co2 inside I am getting about ~300ppm co2.
How is this possible? I cant think of what could be absorbing Co2 inside my house, when I am producing it at a pretty high rate.

I have my HVAC running on a frequent cycle mode also. I have no plants.

The sensor: https://www.sensirion.com/en/enviro...n-dioxide-sensors/carbon-dioxide-sensors-co2/

Here is the data:

Inside:

��������������
 2020/12/05 12:12:22PM 342 2020/12/05 12:12:19PM 346 2020/12/05 12:12:17PM 349 2020/12/05 12:12:14PM 350 2020/12/05 12:12:12PM 353 2020/12/05 12:12:09PM 364 2020/12/05 12:12:07PM 370 2020/12/05 12:12:04PM 372 2020/12/05 12:12:02PM 375 2020/12/05 12:11:59PM 377 2020/12/05 12:11:56PM 378 2020/12/05 12:11:54PM 381 2020/12/05 12:11:51PM 382 2020/12/05 12:11:49PM 384 2020/12/05 12:11:46PM 388

Outside:

������������������
 2020/12/05 11:28:28AM 432 2020/12/05 11:28:26AM 433 2020/12/05 11:28:23AM 434 2020/12/05 11:28:21AM 437 2020/12/05 11:28:18AM 438 2020/12/05 11:28:16AM 440 2020/12/05 11:28:13AM 442 2020/12/05 11:28:11AM 443 2020/12/05 11:28:08AM 444 2020/12/05 11:28:06AM 445 2020/12/05 11:28:03AM 444 2020/12/05 11:28:01AM 446 2020/12/05 11:27:58AM 449 2020/12/05 11:27:56AM 450 2020/12/05 11:27:53AM 450 2020/12/05 11:27:50AM 453 2020/12/05 11:27:48AM 463 2020/12/05 11:27:45AM 464 2020/12/05 11:27:43AM 463

Last edited:

Ivan Seeking
Staff Emeritus
Gold Member
My first guess is that the levels are cycling with the time of day and your levels inside are lagging those outside.

.Scott and FactChecker
My first guess is that the levels are cycling with the time of day and your levels inside are lagging those outside.

im not sure what you mean by that :(

Ivan Seeking
Staff Emeritus
Gold Member
im not sure what you mean by that :(

Take data over a 24 hour period and see how much the levels vary. It may be rising and falling outside, which causes it to rise and fall inside but after a delay.

sophiecentaur
Take data over a 24 hour period and see how much the levels vary. It may be rising and falling outside, which causes it to rise and fall inside but after a delay.

The outside concentration can only reduce the inside concentration if the inside concentration was higher.
because it is higher outside, that can only mean there is something constantly absorbing co2 inside my house or the sensor is defective. I understand there is a correlation between the outside co2 and inside, but I am trying to understand why it is on average lower.

Ivan Seeking
Staff Emeritus
Gold Member
The outside concentration can only reduce the inside concentration if the inside concentration was higher.
because it is higher outside, that can only mean there is something constantly absorbing co2 inside my house or the sensor is defective. I understand there is a correlation between the outside co2 and inside, but I am trying to understand why it is on average lower.

Take data inside and outside over a 24 hour period and I'll bet it all makes sense. ;)

.Scott, sophiecentaur, berkeman and 1 other person
Stephen Tashi
Is the ppm rating independent of the temperature of the gases being sampled?

sophiecentaur, Klystron, BillTre and 1 other person
Is the ppm rating independent of the temperature of the gases being sampled?
It comes with a temp and humidity sensor built in to compensate

Take data inside and outside over a 24 hour period and I'll bet it all makes sense. ;)
Lets say over 24 hours the concentration inside is higher than outside. I still dont understand why the smaller sample size would not match

anorlunda
Staff Emeritus
The outside concentration can only reduce the inside concentration if the inside concentration was higher.
Suppose the outside concentration varies between 200 and 600 though the day, but the inside concentration responds so slowly that it varies only 300 to 500.

CO2 generated or absorbed inside the house could bias the inside reading. However, it's easy to generate C02, and difficult to absorb it.

Stephen Tashi
It comes with a temp and humidity sensor built in to compensate

Perhaps you can think of a simple test to see if that function actually works.

What's the standard definition of "ppm"? Parts-of-what per million what?

Is it a volume measurement ? - volume of CO2 per volume of space?

Is it a fraction-of-volume measurement? - fraction of volume of CO2 per total volume of sample?

Is it a weight measurement? - weight of CO2 in sample per weight of whole sample?

Is it a count of molecules? - number of CO2 molecules in sample per total number of molecules in the sample

Last edited:
Bystander
Homework Helper
Gold Member
However, it's easy to generate C02, and difficult to absorb it.
You're certain? Construction materials? Sheetrock/drywall effects? You're familiar with the (non-)results from "Biosphere 2"?

BillTre
Gold Member
I was thinking exposed amounts of water inside the house could absorb the CO2.
CO2 is highly soluble in water (like open aquariums or inside pools). A humidity sensor would only sense water in the air for its corrections.

But I like the idea of construction materials better.
Lots of those in houses. Not so much, bodies of water.

Some plastics can absorb water to some extent also, but they have limited capacity and would become saturated.

lomidrevo
fresh_42
Mentor
2021 Award
My first thought was a broken device. There is definitely more ##CO_2## inside than outside: breathing or smoking are two sources. As I read about the correction automatism of the device, I thought: what does it correct where to? If this automatism is meant to make different sources comparable, then it might well tune down actual values inside.

An easy test would be to compare two measurements directly after an air exchange, and then after the room is back to its normal temperature.

russ_watters
BillTre
Gold Member
Some things I noticed about the numbers:
1. the time points were fairly tightly clustered in time (not much time for variations to arise, unless due to wind)
2. the numbers seemed to vary quite a bit, but not enough to overlap the two distributions
3. Outside and inside readings both generally trended down over a period of less than a minute
This indicates possible problems/tests WRT the readings:
• Accuracy not down to the tens of PPMs? Find a CO2 standard to test your instrument on (or make one), or get a test based on a different process for comparison.
• There maybe some kind of warm up or equilibration period for the probe in a new condition (some pH meters can be like that); take readings over a longer time period, wait until meter settles down to take more accurate readings. Consult those who made it for details.
On the other hand, such a trend in one direction could be an electronic problem in the sensor system (in pH and I think redox measuring systems, a leak of a current in to the body of water being measured can off-set the readings an electronic sensor like a pH electrode would detect).
• Wind blowing variable air conditions to the sensor (making for a series of different air samples being measured); take a sample of air in a large garbage bag. Take a series of readings of that specific air sample to see whether or not the readings are stable.

Summary:: I purchased a high accuracy IR co2 sensor. Outside it reads ~400 inside I am getting about ~300.

How is this possible? I cant think of what could be absorbing Co2 inside my house, when I am producing it at a pretty high rate.

I have my HVAC running on a frequent cycle mode also. I have no plants.

The sensor: https://www.sensirion.com/en/enviro...n-dioxide-sensors/carbon-dioxide-sensors-co2/

Here is the data:

Inside:

��������������
 2020/12/05 12:12:22PM 342 2020/12/05 12:12:19PM 346 2020/12/05 12:12:17PM 349 2020/12/05 12:12:14PM 350 2020/12/05 12:12:12PM 353 2020/12/05 12:12:09PM 364 2020/12/05 12:12:07PM 370 2020/12/05 12:12:04PM 372 2020/12/05 12:12:02PM 375 2020/12/05 12:11:59PM 377 2020/12/05 12:11:56PM 378 2020/12/05 12:11:54PM 381 2020/12/05 12:11:51PM 382 2020/12/05 12:11:49PM 384 2020/12/05 12:11:46PM 388

Outside:

������������������
 2020/12/05 11:28:28AM 432 2020/12/05 11:28:26AM 433 2020/12/05 11:28:23AM 434 2020/12/05 11:28:21AM 437 2020/12/05 11:28:18AM 438 2020/12/05 11:28:16AM 440 2020/12/05 11:28:13AM 442 2020/12/05 11:28:11AM 443 2020/12/05 11:28:08AM 444 2020/12/05 11:28:06AM 445 2020/12/05 11:28:03AM 444 2020/12/05 11:28:01AM 446 2020/12/05 11:27:58AM 449 2020/12/05 11:27:56AM 450 2020/12/05 11:27:53AM 450 2020/12/05 11:27:50AM 453 2020/12/05 11:27:48AM 463 2020/12/05 11:27:45AM 464 2020/12/05 11:27:43AM 463
Is it a new house and/or with new concrete?

Do you have many house plants?

Is it a new house and/or with new concrete?

Do you have many house plants?

house from 1950, no plants

fresh_42
Mentor
2021 Award
house from 1950, no plants
weed counts as plants

cmb
house from 1950, no plants
A mystery then.

These are very low levels of CO2 for accurate measurements. What is the calibration and precision of the instrument, and what is the temperature offset of the gases it is measuring?

Suppose the outside concentration varies between 200 and 600 though the day, but the inside concentration responds so slowly that it varies only 300 to 500.

CO2 generated or absorbed inside the house could bias the inside reading. However, it's easy to generate C02, and difficult to absorb it.

From my understanding its not possible under standard environmental conditions for the outside air to be under 400ppm co2 since thats the global level for a few decades now. Therefore the outside air could never influence the inside air to be under 400 ppm.

BillTre
Gold Member
Have looked more closely at your numbers.
There are two time separated sets of closely clustered points, at two different locations.

The first (based on the time stamp) set (outside) has a smaller downward (wrt time) slope (-0.688 ppm/s)
than the second set (inside; -1.277 ppm/s).
They both slope down.

The period between the readings (2658 s) is too large account for the overall differences inside/outside just based on either of the slopes, but it is in the same temporal direction.

I suspect something instrumental.
Is this a leave it on all the time type meter or a turn it on and use it for spot checks kind of meter?

OmCheeto
Gold Member
I've never owned nor operated a CO2 device, but the rapidity with which the CO2 levels dropped, and the sequence of the readings, indicates that your batteries might need changing.

Just a guess, of course.

Merlin3189, Rive and BillTre
jrmichler
Mentor
Do one continuous ABAB test: Inside for one hour, outside for one hour, inside, outside. ABAB tests are good for separating sensor drift from actual reading changes.

The datasheet states a 20 second time constant response to a change in ##\rm{CO_2}## concentration, so the test should be long enough for the sensor to stabilize, plus longer to look for concentration fluctuations.

BillTre
Frodo
Gold Member
The data sheet specifies: Accuracy: ± (30 ppm + 3%)

It's a bit ambiguous what that means. If I assume the 3% applies to the measured value and not to the error itself I have, at 400ppm, an accuracy of ± (30ppm plus 12ppm), or ± 42ppm. So, an actual value of 400ppm could display as 358 to 442 ppm.

Also, the measurement range starts at 400ppm so values below that are out of spec. The present global CO2 level is about 412ppm so you are unlikely to get external measurements differing much from that.

Never confuse accuracy with resolution!

Second, I would have thought the outside values to remain fairly stable assuming no close by variable CO2 emitters. What readings do you get far from potential sources?

Apart from addressing the seemingly systematic change @OmCheeto found I think you should also check the functionality of the instrument with some obvious CO2 sources.